Compensating circuit for photographic timers



R. A. CLAPP April 20, 1965 COMPENSATING CIRCUIT FOR PHOTOGRAPHIC TIMERSFiled Nov. 2, 1961 4 Sheets-Sheet 1 FIG.2

T C E J B 0 E C N A W M U L m E NEGA VE T C E J B o E c N A N M U L G 0l- FIG. 3

uztwom u 60 LOG LUMINANGE OBJECT FIG.|

INVENTOR.

ROY A. CLAPP TIVE REFLECTION DENSITY wwawz/ mw POSITIVE ATTORNEYS 4Sheets-Sheet 3 V 24VAC 1 IZOV 0c INVENTOR ROY A. CLAPP ATTORNEYS R. A.CLAPP m azwm/yw COMPENSATING CIRCUIT FOR PHOTOGRAPHIC TIMERS Filed NOV.2, 1961 A ril 20, 1965 2I8R 296R G N %3 G 3 3% l 3 R. A. CLAPP A ril 20,1965 COMPENSATING CIRCUIT FOR PHOTOGRAPHIC TIMERS Filed Nov. 2, 1961 4Sheets-Sheet 4 250 VDC FIG.7

6mm 5 wmnwomxm INVENTOR.

ROY A. CLAPP WZX MM1EJ l mu. INTENSITY ATTORNEYS United States Patent3,178,999 COMPENSATING CIRCUIT FOR PHOTO- GRAPHIC TIMERS Roy A. Clapp,Minneapolis, Minn., assignor to Pako Corporation, Minneapolis, Minn., acorporation of Delaware Filed Nov. 2, 1961, Ser. No. 149,7 8 Claims.(Cl. 88-44) This application is a continuation in part of theapplication of patent of Roy A. Clapp for Photographic Printer, filedJanuary 30, 1959, Serial No. 790,226 now US. Patent No. 3,100,419,issued Aug. 13, 1963.

This invention relates to photographic timers in which the time iscontrolled by the action of a photo-electric cell signal.

An object of the present invention is to provide a photo cell controlledtimer having an extended range suitable for use in printing negativeswhich include both very dense and very thin negatives.

A further object is to provide a timer suitable for printing uponmaterials having non-linear sensitivity characteristics.

Another object is to provide a timer suitable for printing negativeswhich are in themselves non-linear records of the original subjectmatter.

Another object of this invention is to provide a timer for colorprinting in which the timing balance of the color emulsions can bemaintained even when the sensitometric curves differ from each other foreach of the color components.

A still further object of the invention is to provide a timer which isinexpensive, simple and reliable in operation yet flexible in adjustmentso that changes can easily be made :for different kinds of materialwithout the requirement of skilled technical knowledge on the part ofthe operator.

The invention will be best understood from the following descriptionwhen read in connection with the accompanying drawings in which,

FIG. 1 is a combined graph of negative and positive material arranged toshow the tone reproduction when both materials are properly exposed.

FIG. 2 is a combined graph showing the tone reproduction which resultsfrom the printing of an underexposed negative by the usualphoto-electric cell controlled timer.

FIG. 3 is a combined graph illustrating the improvement in tonereproduction which is secured through the use of the invention.

FIG. 4 is a drawing of a schematic circuit of a complete color timerembodying the present invention and corresponding to FIG. 12 in theparent case.

FIG. 5 is a drawing of a schematic circuit of the programming and relaycircuit used in conjunction with the preceding circuit corresponding toFIG. 13 in the parent case.

FIG. 6 is a drawing of a schematic circuit of part of a black and whitetimer embodying the present invention.

FIG. 7 is agraph of time of exposure vs. signal intensity.

FIG. 8 is a drawing of an alternate arrangement of components operatingin the same manner as the circuit shown in FIG. 6.

FIG. 9 is a drawing of another version of the invention.

In reviewing the disclosures contained in the parent application on thisinvention it is found, that while the structure is clearly disclosed anddescribed, the theory of operation is inadequate and thereforesubstantially greater detail will now be presented in order to make 1the function [and operation of this invention clear.

, the curve la-l.

It is old in the art to control printing exposure by arranging aphotoelectric cell to sample the printing light after transmissionthrough the negative being printed and using this photo-electric cellsignal current to charge a capacitor. Since the rate of charge isproportional to the intensity of the printing light and likewise,printing time is assumed to be proportional to such intensity, thisserves as a usual means for controlling exposure time.

This has been particularly unsatisfactory, since with badly over orunder-exposed negatives the best print requires an exposure which is notinversely proportional to the light intensity. In such cases theoperator must modify the automatic timing cycle to overcome thedeficiencies of this system. While these deficiencies are undesirable inblack and white photography, they are even more serious in colorphotography where several emulsion layers are involved and thecharacteristics of these layers are different from one another. It issuggested that reciprocity failure is the cause of this problem andseveral solutions have been proposed to modify the exposure time inaccordance with the Schwarzchild exponent. Such devices have beencomplex and subject to frequent maladjustment and malfunction. Further,it has been found that correction according to Schwarzchild exponent isinadequate in itself.

There are many factors, some known and others un known, which lead toremakes in printing with auto matic controls. Two will be mentionedwhich, as far as known, have not been previously pointed out.

First, while theoretically a timer of the type described should operatein a strictly linear fashion, this is actually not true in practice.Referring to FIG. 7 the abscissa of this graph is the photo-electriccell signal, assumed to be exactly proportional to the printing lightintensity after passing through the negative. The ordinates arecorresponding exposure times. This graph is drawn on a log-10g scale.Curve 1-1 represents a prefect inverse proportion of light versus time.Actually, however, a practical timer tends to behave generally in accordwith The exact form of this curve is dependent on many variablesconcerned with the design and fabrication of the timer itself. But, inmany cases, the timer will fire-out eventually with no photo-electriccell signal at all. The causes include energy emitted from the grid ofthe tube connected to the capacitor tending to charge said capacitor,thermal-junction currents, absorption of the dielectric of thecapacitor, etc. These combine to much more than otfset the leakage lossof the capacitor. Consequently, it is desirable to provide compensationfor such a timer when used for printing dense negatives where the signalis extremely small and the printing time is a long one.

On the other hand, an undenexposed negative (numerically more common) isalso benefitted by compensation for reasons which will now be explainedwith the aid of FIGS. 1-3. These graphs have been constructed generallyin accordance with the method of Jones for the graphic solution ofobjective tone reproduction, which is discussed in Chapter 23, andparticularly on page 913 et seq., revised edition (1954), The Theory ofthe Photographic Process. On the right side of each of these figures acurve corresponding approximately to the sensitometric characteristicsof the negative material is drawn, at the left side of the figures is acurve corresponding approximately to the sensitometric properties of thepositive material.

Referring to FIG. 1, across the top of the first block is a scalecorresponding to the logarithm of the luminance of various portions ofthe object and it is considered for this example that the range of theseluminances extends from A to C in which A corresponds to the area whichhas the least luminance and C corresponds to the area of the objecthaving the greatest luminance. A third line B corresponds to thegeometric mean luminance of this range. These luminances correspond tothe logarithm of exposure scale for the negative which appears on thescale under the negative curve. After development, the negative areaswhich received exposure corresponding to A, yield a density denoted asD. Areas which receive ex posure corresponding to B, yield a densitydenoted as E, while areas which receive exposure corresponding to C,yield a density corresponding to F. The horizontal dotted lines D, E,and F are extended to the left until they intersect the sensitometriccurve of the positive material in the second block. Since the density ofthe negative area determines the exposure of the corresponding areas ofthe positive, it will be clear that the densities, D, E, and Fcorrespond to the log. of exposure of the positive ma terial and whenprocessed the positive material yields reflection densitiescorresponding to I, H and G. Tracing the dotted line C in the firstblock to its point of intersection, thence along the horizontal line F,to the intersection in the second bloclr, and finally to the verticalline G, we see that the areas of highest luminance in the object arereproduced as the lowest reflection density in the positive material, oras nearly white paper. Correspondingly, by tracing the area of leastluminance A in the first block, across by dotted line D to itsintersection and vertical dotted line I, we find that the darkest partsof the object are reproduced as the densest areas of the positive sothat the appearance of the original subject is reproduced in the print.

In the case of a correctly exposed negative and print the range ofexposure is so placed on the negative curve that it is contained withinthe portion of the sensitometric curve that is substantially a straightline. It can also be seen that the mean luminance corresponds to themean density of the positive material so thatB is exactly midway betweenA and C in the first block and the corresponding area of the print H ismid-way between G and I in the second block. In printing such a negativethe photo-electric cell timer is set up so that its target center ismid-way between the extreme densities of the negative D and F and thistimer target is denoted by a circled point labelled TT. The normalposition of the target center is such that it exposes the mean densityof a negative to produce a print density approximately-centered on thesensitometric curve of the positive material. Consequently in FIGS. 1and 2, it appears mid-way between the upper and lower limits of negativedensity and pposite to the center of the curve of the positive material.Thereafter ditterent negatives which are properly exposed are printed inaccordance with the foregoing description.

In FIG. 2, an under-exposed negative of the same sub ject is shown wherethe luminance of the object now occupies the positions A, B, and C, andconsequently intersect the sensitometric curve further down on thecurved portion so that the luminance A is translated into the density D,luminance B results in the density E and the luminance C results in thedensity F. Comparing these with the previous example it will be notedthat the total range of densities in the properly exposed negativeconsists of 23 units, whereas in the under-exposed negative there is adifference in density of only 14 /2 units. Further, the mean luminanceof the properly exposed negative results in a mean density of E, whichis very near the center of the density range, whereas it will be seenthat E is much closer to D than it is to F in FIG. 2. The timer targetsetting denoted by the circled point in FIG. 2 is still half-way betweenthe densities F and D so that the print is automatically exposed inaccordance with FIG. 2 so that the reflection density G whichcorresponds to the positive exposure is no longer nearly white. Thus,the highlight areas of the print are reproduced as much too dark.Further, the middle tone H is much closer to the darkest part of thepicture represented by I so that the fact that both the middle tone andthe highlights are printed darker than the original object results in anunpleasant reproduction.

FIG. 3 shows an improved way of printing such an under-developednegative in which the present invention serves to automatically alterthe target center adjustment whenever an under-exposed negative isencountered. This has the elfect of reducing the exposure of thepositive material so that the density P is printed lighter with areflection density of G, which is the same as that of the normalnegative in FIG. 1. At the same time H is shifted more nearly mid-waybetween G and I, resulting in a print which is much improved andgenerally more acceptable in quality.

This invention secures these results in a very simple Way.

For the purpose of illustrating an application of the invention to aparticular timer, the circuitry shown in FIGS. 4 and S is employed. Thiscircuitry is suitable for use in color printing and employs three timersIR, 16 and 18. These circuits being similar but used for differentcolors, only the timer IR will be described in detail, it beingunderstood that the same reference characters to which the suffixes Gand 8 instead of R will be used to designate the corresponding parts.However structure not directly associated with the instant inventionwill not be described reference to the parent case for such data beingreferred to and forming a part of the instant disclosure as if the samewere described herein.

The Timers The timer IR includes a compensating circuit LR, a triggercircuit QR, an interpolating circuit MR, a density modifying circuit NR,a color modifying circuit OR and a light compensating circuit PR. Thesecircuits will now be described in detail.

One element of each of the switches R, 145G and 145B operating the colorfilters of the printer are connected to a common conductor 210 which inturn is connected to the anode of a tube 148. Tube 148 is energized by apower line 207 which is connected to a source of direct current ofapproximately 1000 volts. This line comprises a conductor 208 which isconnected to the negative side of the power source and a conductor 209which is connected to the positive side and which is grounded. Thecathodes 156 of this tube is connected to one end of a voltage divider153 and to a conductor 211. This conductor in turn is connected to avariable resistor 212 which in turn is connected to the conductor 208.The other end of the voltage divider 153 is connected to a resistor 213which is grounded.

The compensating circuits The compensating circuit LR includes avariable resistor 214R which is connected to a conductor 215R and whichin turn is connected to another conductor 216R. This latter conductor isconnected to the other element of the switch 145R. A condenser 217R isconnected to the conductor 215R by means of a conductor 219R and toanother conductor 218R which in turn is connected to the programmingsection K of the invention as described in the parent case. The movablemember of the resistor 214R is connected by means of a conductor 220R tothe conductor 213R.

The density modifying circuits The density modifying circuit NR includesa single pole three-position switch 221R, the movable contact of whichis connected by means of a conductor 222R to the conductor 226R. Thefixed contacts of this switch are connected by means of conductors 223R,224R, and 225R, to three condensers 226R, 227R, and 228R. Thesecondensers are all connected to a common conductor 229 which isgrounded. All of the switches 221R, 2216 and 221B are ganged and operatein unison.

The bufier circuits The buiier circuit SR includes a tube 231R which may55 be a triode and which has a plate 232R, a grid 233R and a cathode234R. The cathode 234R is connected by means of a conductor 235R to aresistor 236R and which is connected to a grounded conductor 237. Thegrid 233R of this tube is connected by means of a conductor 238R to theconductor 216R.

The interpolating circuits The trigger circuits The trigger circuit QRcomprises a gas filled thyratron tube 245R having a plate 246R, a grid247R and a cathode 248R. A grid resistor 249R is connected to theconductor 242R and to condenser 243R and also to the grid 247R. Theplate 246R is connected by means of a conductor 342R to the programmingcircuitry of FIG. and by means of which the termination of the timingfor the particular color is procured.

The light compensating circuits The light compensating circuit PR isused for selectively printing prints from negatives exposed either byflash illumination or outdoor illumination and comprises twotwo-position single pole manually controlled switches 252R and 253Roperating in conjunction with two potentiometers 254R and 255R. One ofthe fixed contacts of switch 252R is connected to the movable contact ofpotentiometer 254R by means of a conductor 256R. The

' other fixed contact of switch 252R is connected by means of aconductor 257R to the movable contact of potentiometer 255R. The movablecontact of switch 252R is connected by means of a conductor 258R to thecathode 248R or" tube 245R. One end of the resistor of potentiometer254R is connected to one of the fixed contacts of switch 253R while thecorresponding end of the resistor of potentiometer 255R is connected tothe other fixed contact of switch 253R. The other ends of the resistorsof potentiometers 254R and 255R are connected to a common conductor 259which is grounded.

The color modifying circuits The color modifying circuits OR, OG, and OBpermit of varying the intensity of any particular color without changingthe overall density of the print. Power for operating these circuits isderived from a power line 464 connected to a source of direct current ofabout 250 volts. This line comprises a conductor 405 which is connectedto the positive side of the source of power and a conductor 406 which isconnected to the negative side and which is grounded. The circuits OR,06 and OB include otentiometers 261R, 26113 and 261G. One end of each ofthese potentiometers is connected to a conductor 329 which in turn isconnected to the positive side 495 of the line 404. The other ends ofthese potentiometers are connected through conductor 264R, 26413 and264G to resistors 265B, 265R and 265G. These resistors are all groundedthrough a conductor 266. The circuits utilize three single polethree-position switches for each color designated by the numerals 267R,267B, 267G, 268R, 26813, 2686, 269B, 2696 and 269R. The switches 267R,268G and 269B are ganged and moved in unison. Similarly, the switches267B, 263R and 26%} are ganged and also the switches 267G, 268B and 269Rare ganged. The switches 267R, 268R and 269R have connected to the fixedcontacts of the same resistors 271R, 272R, 273R, 274R, 275R and 276R.Similar resistors are connected to the other switches. The resistor 271Ris connected through a conductor 277R to the movable contact of thepotentiometer 261R. The movable contact of the switch 267R is connectedby a conductor 278R to the movable contact of switch 268R. The resistor274R is connected by a conductor 279R to the movable contact of theswitch 26R. The resistor 276R is connected by a conductor ZfiiR to themovable contact of the switch 253R. When any of the three gangs ofswitches are moved from one position to another to increase or decreasethe resistance in one of the color circuits, the resistance in each ofthe other color circuits is decreased or increased by an amount equal toone-half of the increase or decrease in the denoted circuit, thusmaintaining the density the same.

In FIG. 6, the same numerals denote the same parts as in FIGS. 4 and 5,however, the letters R, G, and B have been dropped since this structureis for use with black and white photography, and it is considered thatthis timer is to be operated with a steady state DC. signal.

The circuit shown in FIG. 6 includes a trigger circuit Q comprising athermionic tube 245, having a plate 246, a cathode 248 and a grid 247.The plate 246 is connected to a relay coil 334 by a conductor 342. Therelay coil 334 is connected to a switch 327 having a contact 328connected to the positive terminal of a DC. power supply. The cathode248 of said tube is connected by a conductor 258 to an adjustablevoltage divider 254 arranged between the positive lead of the same DC.power supply and ground, thus providing an adjustable bias potential forsaid tube. The photo-electric cell signal current is applied via aconductor 216 through an adjustable compensating resistor 214 and aconductor 230 to a summing condenser 227, the other terminal of which isgrounded. A normally closed pair of contacts, 295 and 296 maintain thesumming condenser in a shorted condition via conductors 218 and 297. Ashaping condenser 21'7 shunts the compensating resistor 214. Forpurposes of explanation the connection is shown as a dotted conductor219. The compensation circuit comprises said resistance 214 andcondenser 217 and a con ductor 591 connects said resistor to the grid247 of the tube 245.

To best understand the action of the invention, let us assume for themoment that the shaping condenser 217 is omitted from the circuit.Values will now be assigned to the components so that the operation ofthe circuit may better be understood. Assume the capacity of the summingcondenser 227 to be one microfarad. The normal bias to fire the triggertube may be varied by means of potentiometer 254. The signal from thephoto cell is under stood to range between one and eight microamperesdepending upon the negative. The value of the compensation resistor isalso variable between zero and one megohm. In order to show the actionmathematically we will assume the timer to be theoretically linear. Fromexamination of the schematic FIG. 6, it is clear that after the switchpoints 295 and 296 are opened the photo-cell signal current flowingthrough compensating resistance 214 will cause a voltage to appearacross it of a magnitude determined by Ohms Law. Further, the samecurrent will be charging the summing condenser 227 so that an additionalpotential appears across this condenser at a rate proportional to themagnitude of the signal and with the passage of time. It is furtherclear that these two voltages are added together and applied to the grid247 of the tube 245. In other Words, the trigger circuit will fire whenER+EC is equal to EB, where ER is the voltage developed across thecompensating resistor 214 and EC is the voltage developed by thecharging of the condenser 227 and EB is the firing voltage of thetrigger circuit. Since, in the present case, the value of this condenseris one microfarad, it follows that the time in seconds is equal to EB-ER'3" where I is the photo cell signal. Using the above information atable can now be constructed for three sets of conditions, selectedarbitrarily.

FIG. 7 is procured by plotting the data from the three tables as shownin FIG. 7 using log-log scale. Column A shows the timer without aresistor in series with the condenser 227 resulting in a straight line1-1 and simply represents the ideal timer without compensation. The dataof column B is plotted to form curve 2-2 while the curve 3-3 representsthe data of column C. It should now be clear that by varying R and thebias voltage, a wide variety of curves can be generated and while thevalues tabulated cause these curves to cross at point r, this can beeasily altered if desired by a suitable choice of values for the biasvoltage and the resistor. However, assume that a curve is desired to runfrom 2 to r as shown, but without having the curve drop as abruptlybeyond point r as it does under the conditions of column B. By shuntingresistor 214 with the shaping condenser 217 by connecting it withconductor 21? shown dotted in FIG. 6, an R-C network is formed so thatthe voltage across resistor 214 can no longer rise instantly in responseto the photo-electric cell signal; instead it now increasesexponentially until ERC-l-EC equals EB where ERC is the instantaneouspotential across the R-C network. Under the new arrangement both ERC andEC, individually change with time, ERC varies exponentially to rapidlyapproach a limit, while EC varies linearly with time. For example, byusing a 0.2 microfarad condenser 217 connected across the resistor 214as shown by the dotted lines in FIG. 6, it will be found that while theportion of the curve between 2 and r remains substantially unchanged forsmall signals, the curve to the right of r follows closely r-3. Inpractice, by simply selecting the value of the two elements 217 and 214together with appropriate alteration of the firing bias, the timingcurve can be modified to suit any of the present photographic materials.However, it is not necessary to be restricted to ordinary linearresistance, we can use a variety of combinations, linear resistance andnonlinear resistances such as thyrite or crystal diodes, etc.

It is equally possible to insert the compensation circuit between thesumming capacitor and ground as shown in FIG. 8. In this version thecompensation circuit L, comprised of the resistor 214 and the shapingcondenser 217, is in series with the summing condenser 227 which isnormally maintained in a discharged condition by being shorted by closedpoints 295 and 29-6. When initiated, said points open and the photo cellsignal is received via conductor 216. The combined voltage across thesumming condenser 227 and resistor 214 is applied via conductor 501 togrid 247 of trigger circuit 245, the grid 248 being biased via conductor258 and the plate v24f being connected to associated equipment byconductor 342.

A further variation of this invention is shown in FIG. '9 in which thecompensation circuit is composed of two resistors 214a and 2145 arrangedin series with each other and in series with summing condenser 227,resistor 21% being shunted with a shaping condenser 217.

Networks of these kinds with one or more condensers give practically afree choice to the curves developed.

arses-o Thus, it can be seen that the curve to the left of r can notonly be used to correct for the natural drop of the real timer at 1a,but can also over-compensate for this end of the curve, while adilferent compensation of the curve to the right of r can shift theprinting for underexposed negatives in the toe of the print materials aswas previously describe. All this is accomplished by a few inexpensiveparts of great reliability.

So far this invention has been described as it is related to a black andwhite printer, having a single timing channel, but it is obvious thiscan be readily used in exactly the same Way in a color printer, whichhas three separate timing channels and that the component values mightbe varied between the individual channels so as to conform to the natureof photographic materials. A suitable circuit for use in a color printeris shown in FIGS. 4 and 5.

It is equally true that while the conditions which have been used todescribe the action of this invention have been confined to a steadystate photo-electric signal, that by suitable choices of values it maybe also used with a timer which employs pulse signals, in which case theshaping condenser serves a second purpose by reducing the height of thetransient voltages developed during the receipt of the pulse itself.

Changes in the specific form of the invention, as herein described, maybe made within the scope of what is claimed without departing from thespirit of the invention.

Having described the invention, what is claimed as new and desired to beprotected by Letters Patent is:

1. In exposure control apparatus for use in photographic printing, asumming condenser, light sensitive means connected to said summingcondenser and acti vated in accordance with printing intensity, saidlight sensitive means providing a signal current, initiating theexposure and controlling the rate of charge of said summing condenser, acompensating resistor in series with said condenser, the voltage acrosswhich being dependent upon the signal current, a shaping condensershunting said resistor and forming therewith an R-C network, andexposure terminating means energized by the combined voltage of thesumming condenser, shaping condenser and the resistor and operating whenthe combined voltage reaches a predetermined value to terminate theexposure.

2. In exposure control apparatus for use in photographic printing, asumming condenser, a photo electric cell connected to said condenser andactivated in accordance with the printing intensity, said photo electriccell providing a current, initiating the exposure and controlling therate of charge of said condenser, a resistor connected in series withsaid condenser, the voltage across which depending solely upon thesignal current, and exposure terminating means energized by the combinedvoltage of the summing condenser and the resistor and operating when thecombined voltage reaches a predetermined value to terminate theexposure.

3. In exposure control apparatus for use in photographic printing, asumming condenser, light sensitive means connected to said summingcondenser and activated in accordance with printing intensity, saidlight sensitive means providing a signal current, initiating theexposure and controlling the rate of charge of said summing condenser,compensating means including two resistors connected in series with oneanother and with said summing condenser, the voltage across which beingdependent solely upon the signal current, a shaping condenser connectedacross said one of said resistors and exposure terminating meansenergized by the combined voltage of the summing condenser and theresistor and operating when the combined voltage reaches a predeterminedvalue to terminute the exposure.

4. In exposure control apparatus for use in photographic printing, asumming condenser, light sensitive means connected to said summingcondenser and activated in accordance with printing intensity, saidlight sensitive means providing a signal current, initiating theexposure and controlling the rate of charge of said summing condenser, anon-linear resistive element in series with said condenser, the voltageacross which being dependent solely upon the signal current and exposureterminating means energized by the combined voltage of the summingcondenser and the resistive element and operating when the combinedvoltage reaches a predetermined value to terminate the exposure.

5. In exposure control apparatus for use in photographic printing, asumming condenser, light sensitive means connected to said summingcondenser and activated in accordance with printing intensity, saidlight sensitive means providing a signal current, initiating theexposure and controlling the rate of charge of said summing condenser,compensating means including a plurality of resistors connected inseries with one another and with said summing condenser, the voltageacross said resistors being dependent solely upon the signal current,certain of said resistors being shunted by said shaping condenser meansand exposure terminating means energized by the combined voltage acrossthe summing condenser and the resistors and operating when the combinedvoltage reaches a predetermined value to terminate the exposure.

6. In exposure control apparatus for use in photographic printing, asumming condenser, light sensitive means connected to said summingcondenser and activating in accordance with printing intensity, saidlight sensitive means providing a signal current, initiating theexposure and controlling the rate of charge of said summing condenser,compensating means including a plurality of timing networks connected inseries with one another and with said summing condenser, the voltageacross which being dependent solely upon the signal current, andexposure terminating means energized by the combined voltage of thesumming condenser and the timing networks and operating when thecombined voltage reaches a predetermined value to terminate theexposure.

7. In exposure control apparatus for use in photographic printing, anelectron discharge device having a grid and an anode, a photo-multiplierhaving an output element and activated in accordance with printingintensity, means for applying a voltage to said photo-multiplier, anoutput circuit connected to the output element of the photo-multiplierand having a timing branch circuit connected to ground and a grid branchcircuit connected to the grid of said electron discharge device, asumming condenser in said timing branch circuit adapted to be charged bythe photo-multiplier, a variable resistor in said timing circuit inseries with said condenser and subject only to the current produced bysaid photomultiplier and a discharge circuit having a switch therein andparalleling said condenser.

8. In exposure control apparatus for use in photographic printing, anelectron discharge device having a grid and an anode, a photo-multiplierhaving an output element and activated in accordance with printingintensity, means for applying a voltage to said photo-multiplier, anoutput circuit connected to the output element of the photo-multiplierand having a timing branch circuit connected to ground and a grid branchcircuit connected to the grid of said electron discharge device, asumming condenser in said timing branch circuit adapted to be charged bythe photo-multiplier, a variable resistor in said timing circuit inseries with said condenser and subject only to the current produced bysaid photo-multiplier, said resistor being adjustable to one megohm anda discharge circuit having a switch therein and paralleling saidcondenser.

References Cited by the Examiner UNITED STATES PATENTS 2,158,903 5/ 39Knobel 8824 2,561,535 7/51 Paulet et a1. 88--24 2,605,447 7/52 Troup88-24 2,607,266 8/52 Rabinowitz 88-24 2,611,809 9/52 Lee 8824 X2,952,780 9/60 Rogers 88-24 X 3,056,332 10/62 Beregowitz 88-24 3,076,3782/63 Biedermann et al. 8824 NORTON ANSHER, Primary Examiner.

EMIL G. ANDERSON, Examiner.

1. IN EXPOSURE CONTROL APPARATUS FOR USE IN PHOTOGRAPHIC PRINTING, ASUMMING CONDENSER, LIGHT SENSITIVE MEANS CONNECTED TO SAID SUMMINGCONDENSER AND ACTIVATED IN ACCORDANCE WITH PRINTING INTENSITY, SAIDLIGHT SENSITIVE MEANS PROVIDING A SIGNAL CURRENT, INITIATING THEEXPOSURE AND CONTROLLING THE RATE OF CHARGE OF SAID SUMMING CONDENSER, ACOMPENSATING RESISTOR IN SERIES WITH SAID CONDENSER, THE VOLTAGE ACROSSWHICH BEING DEPENDENT UPON THE SIGNAL CURRENT, A SHAPING CONDENSERSHUNTING SAID RESISTOR AND FORMING THEREWITH AN R-C NETWORK, ANDEXPOSURE TERMINATING MEANS ENERGIZED BY THE COMBINED VOLTAGE OF THESUMMING CONDENSER, SHAPING CONDENSER AND THE RESISTOR AND OPERATING WHENTHE COMBINED VOLTAGE REACHES A PREDETERMINED VALUE TO TERMINATE THEEXPOSURE.